WASHINGTON - U.S. officials say efforts have stalled to capture about a dozen people secretly charged in the 2012 attack on the American compound in Benghazi that claimed the lives of Ambassador J. Christopher Stevens and three other Americans.

The individuals have been charged in sealed criminal complaints filed in federal court by the U.S. Attorney's Office for the District of Columbia. They include one of the suspected ringleaders of the attack, Ahmed Abu Khattala, a militia leader with ties to al-Qaeda, said several U.S officials, speaking on the condition of anonymity because they were not authorized to discuss the case publicly.

So far, none have been brought to trial and the lack of progress in capturing Khattala has frustrated U.S. intelligence officials and lawmakers who want to see him and the others prosecuted. One official said that Khattala continues to operate in eastern Libya with impunity.

"He's as free as a bird," said the official.

In an interview, Rep. Mike Rogers (R., Mich.), who chairs the House intelligence committee, said the United States was not making a strong enough effort to capture Khattala.

"I don't believe we have adequately resourced or operationally planned to remove Khattala or those involved in the Libyan 9/11 attacks," Rogers said. "There are consequences for our lack of pursuit. . . . You have removed the notion of certainty in deterrence and you cannot lose that."

FBI Director James Comey has said that capturing suspects in the Benghazi case is a top priority for the FBI.

"It was the first matter I was briefed on which reflects the commitment within the FBI to that case and to bringing to justice those who were involved in the killing of our four folks," Comey said in an interview with reporters a few weeks after he became director in September.

Law enforcement officials said that the United States might have missed its best chance to arrest Khattala earlier this year. The U.S. intelligence community hatched a plan to snatch Khattala and an accused al-Qaeda operative, Nazih Abdul-Hamed al-Ruqai, known by his alias Anas al-Libi. The planning took months, requiring coordination among the FBI, the CIA and the Army's elite Delta Force.

Libyan Prime Minister Ali Zeidan gave the go-ahead to grab Khattala and Ruqai, one U.S. intelligence official said. The plan involved nabbing Ruqai in Tripoli first and then Khattala, with both operations occurring within days of each other.

Ruqai was seized outside his home in Tripoli by U.S. military forces and taken out of the country to a U.S. warship for questioning. He was eventually flown to New York to face charges for the 1998 embassy bombings in East Africa.

The blowback was perhaps fiercer than expected. Days after Ruqai was seized, Zeidan was abducted but eventually released unharmed.

The Khattala mission was scrapped, and now any plans to remove him from Libya are on the back burner.